1. Field of the Invention
This invention relates to a piezoelectric film. This invention also relates to a process for producing a piezoelectric film. This invention further relates to a film forming apparatus for carrying out the process for producing a piezoelectric film.
2. Description of the Related Art
Piezoelectric devices provided with a piezoelectric body, which has piezoelectric characteristics such that the piezoelectric body expands and contracts in accordance with an increase and a decrease in electric field applied across the piezoelectric body, and electrodes for applying the electric field in a predetermined direction across the piezoelectric body have heretofore been used as actuators to be loaded on ink jet type recording heads, and the like.
As piezoelectric body materials, there have heretofore been known composite oxides having a perovskite structure, such as lead zirconate titanate (PZT). The composite oxides having the perovskite structure are ferroelectric substances, which have spontaneous polarization characteristics at the time free from electric field application. With the conventional piezoelectric devices, ordinarily, an electric field is applied in a direction matched with a polarization axis of the ferroelectric substance, and a piezoelectric effect extending in the direction of the polarization axis is thereby utilized. Specifically, heretofore, it has been regarded to be important that material design be made such that the direction of the electric field application and the direction of the polarization axis may coincide with each other (i.e., polarization axis=direction of electric field application).
Heretofore, in order for the spontaneous polarization of the piezoelectric body to coincide with the direction of the electric field application, polarization processing has been performed on the piezoelectric body. For example, in Japanese Unexamined Patent Publication No. 6(1994)-085345, there is proposed a polarization method for a PZT type piezoelectric device, wherein a high voltage is applied across a piezoelectric body taking on the form of a bulk body, while the piezoelectric body is being heated to a temperature close to a Curie temperature.
Also, for example, in Japanese Unexamined Patent Publication No. 2005-262108, there is proposed a method of forming a piezoelectric film with an aerosol deposition technique (hereinbelow referred to as the AD technique), wherein an electric field is formed within a film forming chamber during film formation, and wherein the polarization processing is thereby performed at the same time as the film formation.
Examples of piezoelectric strains include the following:    (1) an ordinary electric field-induced piezoelectric strain of the ferroelectric substance, which strain is caused by the increase and decrease in applied electric field to undergo the expansion and contraction in the direction of the electric field application in cases where a vector component of a spontaneous polarization axis and the direction of the electric field application coincide with each other,    (2) a piezoelectric strain, which is caused to occur when the polarization axis is reversibly rotated by an angle different from 180° due to the increase and decrease in applied electric field,    (3) a piezoelectric strain, which is caused to occur in cases where the crystal is caused by the increase and decrease in applied electric field to undergo phase transition, and a volume change occurring due to the phase transition of the crystal is utilized, and    (4) a piezoelectric strain, which is caused to occur by the utilization of an engineered domain effect, wherein a large strain is capable of being obtained in cases where a material having characteristics, such that the material is caused by electric field application to undergo phase transition, is utilized, and in cases where a crystal orientation structure is set so as to contain a ferroelectric phase having the crystal orientational characteristics in the direction different from the direction of the spontaneous polarization axis (in cases where the engineered domain effect is utilized, the actuation may be carried out under the conditions under which the phase transition may occur, or under the conditions such that the phase transition may not occur).
In cases where each of the piezoelectric strains (1), (2), (3), and (4) described above is utilized alone, or two or more kinds of the piezoelectric strains (1), (2), (3), and (4) described above are utilized in combination, a desired level of the piezoelectric strain is obtained. Also, as for each of the piezoelectric strains (1), (2), (3), and (4) described above, in cases where the crystal orientation structure in accordance with the principle of the strain generation corresponding to the piezoelectric strain is employed, an enhanced level of the piezoelectric strain is obtained. Therefore, in order for high piezoelectric performance to be obtained, the piezoelectric body should preferably have the crystal orientational characteristics.
However, with the bulk piezoelectric device as described in Japanese Unexamined Patent Publication No. 6(1994)-085345 and with the piezoelectric film obtained from the crystal growth by the AD technique as described in Japanese Unexamined Patent Publication No. 2005-262108, it is not always possible to obtain high crystal orientational characteristics, and it is not always possible to obtain sufficiently high piezoelectric performance even if the polarization processing is performed.
The inventors conducted extensive research on a piezoelectric film having good piezoelectric characteristics. As a result, as for a PZT type piezoelectric film obtained by performing film formation on a substrate by use of a vapor phase growth technique, such as a sputtering technique, the inventors succeeded in obtaining a piezoelectric film having markedly high orientational characteristics, wherein the spontaneous polarization ordinarily heads from the substrate side toward the piezoelectric film surface (in the upward direction), without particular polarization processing immediately after the film formation being performed, depending upon the composition and the dopant material.
In cases where the piezoelectric film described above is to be used as a piezoelectric device, a bottom electrode layer may be formed previously on a substrate, the piezoelectric film may be formed on the bottom electrode layer, and thereafter a top electrode layer may be formed on the piezoelectric film. In the cases of the piezoelectric film described above, the direction of the spontaneous polarization heads from the substrate side (i.e., the bottom electrode side) toward the piezoelectric film surface (i.e., the top electrode side). In order for the piezoelectric film to be actuated as the piezoelectric device (i.e., in order for the piezoelectric effect to be obtained), it is necessary for a voltage to be applied such that an electric field is formed in the direction identical with the direction of the spontaneous polarization. Therefore, in such cases, it may be considered that the bottom electrode is grounded, and that an actuation driver for the negative electrode is used so as to apply the negative voltage to the top electrode side.
However, the actuation driver for the negative electrode is not used widely. Therefore, the problems are encountered in that only a few kinds of the actuation drivers for the negative electrode are available and in that the cost of the actuation driver for the negative electrode is higher than the cost of the actuation driver for the positive electrode. In cases where the bottom electrode is subjected to patterning processing and is taken as the address electrode, and the top electrode is taken as the ground electrode, the actuation driver for the positive electrode, which actuation driver has been used widely, is capable of being utilized. However, in such cases, the production process is not capable of being kept simple.
Therefore, such that the actuation is capable of being performed by use of the actuation driver for the positive electrode, reverse polarization processing for reversing the direction of the polarization has been performed on the piezoelectric film having been formed on the substrate, which piezoelectric film has the high orientational characteristics and having the spontaneous polarization heading from the substrate side toward the piezoelectric film surface. The piezoelectric film has thus been prepared, which piezoelectric film is capable of being actuated such that the substrate side is grounded and such that the piezoelectric film surface side is set at the positive potential. As the method for the polarization processing, the method as described in Japanese Unexamined Patent Publication No. 6(1994)-085345 has been employed, wherein after the piezoelectric film has been formed, the piezoelectric film is taken out into the ambient atmosphere and heated together with the substrate to a temperature in the vicinity of the Curie temperature, and wherein an electric field is applied across the piezoelectric film. However, it has been found that the problems are encountered in that the reverse polarization degree is low, and in that depolarization is apt to occur.
The method described in Japanese Unexamined Patent Publication No. 2005-262108, wherein the electric field is applied at the time of the film formation with the AD technique, is not applicable principally to the cases wherein the film formation is performed with the sputtering technique.